Caption: Influenza C virus. Coloured transmission electron micrograph (TEM) of the influenza C virus, which is one of the causes of influenza (flu) in humans and pigs. It is rare compared to types A and B, but can be severe. The virus particles have an outer coat bearing the proteins haemagglutinin and neuraminidase, which allow the particles to enter the host's cells and reproduce. Contracting the flu usually causes lethargy, aching muscles, fever and nausea.